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John Harrison was way ahead of his time when he built a clock accurate to one second per month 278 years ago. Although his work on domestic clocks was outstanding, he is best remembered for solving the problem of measuring longitude whilst at sea. Harrison's methods went against the established view that astronomy, through star maps and the position of the moon, could solve the longitude problem. He believed and proved that accurate clocks held the answer. His work on sea clocks paved the way for today's global positioning systems that use atomic clocks with astonishing levels of accuracy. On 24th March this year, Harrison's birth and death date, a memorial tablet commemorating his life and works was unveiled in Westminster Abbey a long overdue acknowledgement of his key role in accurate time measurement.

The Royal Greenwich Observatory was set up in 1675 to map the heavens with a view to determining longitude at sea. To further focus scientific attention on this matter the1714 Longitude Act made £20,000 prize money available to anyone who could solve the problem. The Royal Society had key roles in both initiatives as the President of The Society appointed the Astronomer Royal in charge of the Observatory, and the Longitude Commission that oversaw the Act, was largely made up of Fellows. The Society was supportive of Harrison's alternative efforts with clocks despite the many members who were astronomers.

For example in 1741, a letter to the Longitude Commission signed by twelve Fellows ensured Harrison received further funds to continue his work. Nine of the twelve signatories had astronomy connections, including the first and second Astronomers Royal. Further support for Harrison came when he was awarded the Society's Copley Medal in 1749, 'On account of those very curious instruments, invented and made by him, for the exact mensuration of Time'.

Harrison eventually proved that a clock of his design was accurate enough to determine longitude at sea when in 1761 his son William took his H4 clock (actually a big watch) on the sea trial required by the Longitude Commission. Despite being accurate to within two miles by the time the ship reached Jamaica, the Commission refused to make the award and it was not until 1773, just three years before his death, that Harrison finally received the rest of his money by way of the intercession of King George the Third.

'The excitement of time' exhibit goes on to chart the progress of time measurement up to the present day, including the phenomenal accuracy achieved with atomic fountain clocks that is making it likely that New Physics will be discovered.